A phase-locked loop is an electronic circuit used to lock an oscillator in phase with an arbitrary input signal. Typically, phase-locked loops comprise a phase comparator, a low pass filter and an error amplifier in the forward signal path with a voltage controlled oscillator (VCO) in the feedback path. The function of the VCO is to produce a signal which will act as a reference to the phase comparator, and also track any phase change in the input signal. The frequency of the VCO output is controlled by a dc error signal which is generated by the phase comparator. The dc error signal generated is proportional to the difference in the phase angle between the VCO and the reference input signal.
An important performance parameter of a phase locked loop is its hold-in range. The hold-in range is the maximum change in input frequency for which the loop will remain locked. It is governed by the dc gain of the loop. As the frequency of the input signal is changed, the differential in phase of the two signals inputted to the phase detector will produce a dc voltage which will change the output frequency of the VCO. As the input frequency is further altered, the phase angle differential will continue to increase until it reaches a point where the loop unlocks.
Phase lock loops have numerous applications. While phase locked loops are perhaps most widely utilized in television receivers, they can be adapted to almost any application requiring frequency demodulation. It is well known, for example, that a rigid structure such as a cantilevered beam will vibrate at a characteristic resonant frequency. This frequency is dependent upon the length, width, geometric and structural characteristics of the member with respect to the specific point or node where it is held rigid. In the case of a fixed cantilevered beam, maximum oscillatory deflection will occur at the free end. By damping the free end of a vibrating cantilevered beam, the resonant frequency of vibration will increase and the phase of vibration of the structure will be shifted positive.
A vibrating rigid member can be utilized to produce an input signal for a phase-locked loop circuit. By utilizing one output of the VCO as a driving signal, a rigid member can be made to vibrate at a characteristic frequency. An input signal generated directly from the driven rigid member can then be utilized as an input to a phase comparator in the phase-locked loop. If the vibrating member is deflected or damped, thereby increasing its frequency, the phase of the input signal fed to the phase comparator will change. This phase change will produce a positive output on the phase comparator which will generate a corrective dc error signal to the VCO. The VCO will accordingly attempt to track the change in input phase. If the phase differential ultimately exceeds the hold-in range of the circuit, the loop will unlock.
Piezoelectric films generally have been thought to be unsuited for sensing constant or slowly changing conditions such as the phase changes characteristic of phase-locked loop circuits. New circuit techniques, however, permit piezoelectric films to detect steady-state and slowly changing conditions. These new techniques permit the film to be utilized in association with a vibrating member for use in a phase-locked loop circuit.
Kynar.RTM. piezoelectric film, a commercially available product of Pennwalt Corporation, Philadelphia, PA, assignee of the present invention has excellent electro-mechanical properties which make it ideal as a transducer material for use in association with a vibrating rigid member. When a voltage of proper polarity is applied to a sheet of piezoelectric film, the film becomes thinner and elongates. If a voltage of opposite polarity is applied, the film accordingly contracts (or shortens) and simultaneously thickens. Accordingly, if directly adhered to a rigid member, the electro-mechanical property of Kynar.RTM. piezo film can be utilized to generate mechanical vibration in the rigid member upon application of an ac signal.
Correspondingly, the mechano-electrical properties of Kynar.RTM. piezo film make it ideal as a transducer for generating an ac input signal for the phase-locked loop. When an external force (such as the mechanical vibration induced in a rigid member) produces compressive and tensile strain in a second film, an electric potential proportional to the change in mechanical stress is produced across the film. When the direction of force is reversed, the polarity of the resulting voltage is reversed. Hence, the vibrations of the beam generated by a first film results in an alternating voltage in a second film which can be used as an input signal in a phase locked loop circuit. Kynar.RTM. piezoelectric film is sensitive enough such that stretching of a mere 5-10 micrometers can yield voltage outputs ranging from 10-1000 volts.
It is an object of the present invention to utilize Kynar.RTM. piezo film in order to generate vibration in a rigid structural member and to utilize that vibration to produce a corresponding input signal for a phase locked loop circuit.
It is another object of the present invention to utilize Kynar.RTM. piezo film in order to generate an input signal for a phase-locked loop circuit which can be used for practical applications such as in an alarm system or a liquid level detector.